This invention relates to the application of coatings to substrates, and, more particularly, to plating on titanium.
Titanium is a metal of great interest in the aerospace industry, because of its combination of good mechanical properties, low density, and operability in a number of special forming processes. Titanium is widely used in applications requiring high strength at moderate temperatures, such as skin structures, primary load bearing members, and fasteners, for example. A variety of titanium alloys are available, and the term "titanium" as used herein is intended to include the pure metal as well as its various alloyed forms.
In some of its applications, it is desirable that a titanium piece be coated or plated to achieve improved corrosion or oxidation resistance, increased surface hardness or galling resistance, improved dimensional sizing, or for other surface-related reasons. For over 30 years, there have been proposed methods for applying adherent coatings to titanium, but for the most part the methods have proved to be inoperable. The plating of titanium alloys with metals such as nickel remains as a problem, and improved methods are required for such plating.
Titanium alloys are difficult to plate with adherent metal coatings because they form a tenacious, passive oxide film quickly. The oxide film may be removed by various etching procedures, but the oxide film reforms so rapidly that it is difficult to accomplish any coating before the film reforms to block access of the plated atoms to the surface. If the plating is accomplished over the oxide film, a layer of metal can be deposited, but the layer is not sufficiently adherent for most purposes. Bending of the titanium piece causes the coating layer to debond from the surface, rendering the layer useless for its intended purposes.
There is therefore a continuing need for a method of coating metals such as electroless nickel onto titanium, particularly its alloys. The present invention fulfills this need, and further provides related advantages.